1. Field of the Invention
The present invention relates to graphic tablets, and more particularly, to a tablet of the type incorporating grid wires therein and positioned beneath the surface of a tablet for receiving signals radiated from a pen.
2. Prior Art
A great variety of devices have been disclosed in the prior art for ascertaining the position of a pen on a graphic surface. These prior art techniques include the generation and radiation of electromagnetic waves as well as the radiation of electric waves. Typically, the prior art includes a means for applying power to the tablet with the pen being used as a receiving antenna for detecting the presence of a radiated signal in time and phase related to the generation of the signal in the tablet. For example, U.S. Pat. No. 3,767,858 incorporates the generation of signals supplied to the grid wires in the tablet; the signals supplied to the grid wires vary in frequency and phase. The electro-static field is sensed by the pen and is converted into an output that corresponds to the physical position of the pen apparatus. The apparatus described in this patent includes the use of two separate signal frequencies to separate the composite signals picked up by the pen; the signals are subsequently processed by separate phase-locked loops.
U.S. Pat. No. 3,851,097 discloses apparatus employing a phase-shifting technique similar to that described in U.S. Pat. No. 3,767,858, and uses phase-locked loops and frequency phase-shifts as a means for converting the position of the pen on the surface into corresponding electrical output signals. The system operates by the detection of the phase shift that occurs across the graphic surface in both X and Y directions by means of a phase-locked loop to detect the difference between the signal picked up by the pen and a reference frequency and phase source.
U.S. Pat. No. 3,886,311 discloses a pen and corresponding tablet surface employed for graphic communication. A pen capacitively picks up an electro-static field from the surface for the purpose of sensing location of the pen relative to the electrostatic field. The position of the pen is determined in terms of X and Y coordinates and provides an output signal in response to the position of the pen. The apparatus also discloses means for loading a writing element or ballpoint cartridge element by a side-loading method and discloses the use of the ballpoint element for writing on paper that may be placed on the graphic surface.
U.S. Pat. No. 3,983,222 discloses a method and apparatus for converting the position of a pen on a surface into corresponding output signals. The system involves the use of a phase-shift across the surface employing an electrostatic field for transmission to a receiving pen. The pen, in turn, provides an output signal to circuitry that resolves the position of the pen on the surface. Specifically, the system involves the use of phase-shift and frequency change in a servo system incorporating a phase-locked loop and error detector to determine the displacement of the pen from a central location on the surface of the tablet. In addition, the apparatus references the frequency and phase of the signal picked up by the pen with an internal reference in the phase-locked loop and includes a bandpass filter element that converts frequency shift into phase shift. The system operates in a servo mode in which the pen movement away from its center position, in both X and Y directions, creates an error signal that results in a frequency change in a phase-locked loop. The frequency changes in such a fashion as to attempt to minimize the net phase-shift at any position in which the pen may be located. It constitutes an electrostatic closed-loop feedback means of pen position detection with respect to the apparatus of U.S. Pat. Nos. 3,878,858, 3,886,311, 3,851,097.
U.S. Pat. No. 4,022,971 discloses another graphic communication system employing X and Y conductors in the graphic surface that generate an electrostatic field; the resultant electric field is sensed by a pen employing a phase-shift and a servo system that is substantially the same as those described in the previously discussed patents. However, the apparatus also employs a means to switch the location of the active grid elements in the surface from location to location so that the actual active area of the surface is limited to a small portion of the total graphic surface. Multiplexing circuitry is employed, in addition to the basic graphic methods previously described, to move the writing surface essentially from one position to another so that the pen is always located over an active surface area. The apparatus employs an electric signal fed to grid elements consisting of X and Y conductors that are sensed by the pen; by means of measurement of error detection of frequency and phase-shift an output signal is created that corresponds to the pen location on the total surface.